Typically, an optical fibre transmission system includes a plurality of individual transmission paths, each of which is constituted by a plurality of optical fibres connected together (for example by splicing or fusion). Problems arise with the maintenance of such a system, particularly when a given transmission path has an unacceptable power loss. In such a case, it may be necessary to measure the power transmission in a large number of the optical fibres constituting that transmission path. Known optical power meters require a cleaved fibre end for power measurement. This requirement entails not only disruption of the system, but it is time-consuming and requires the use of special tools and a high level of skill by the operator.
There are also known various devices and arrangements which utilise the known fact that if an optical fibre is bent through a sufficiently small radius, light carried by the optical fibre can escape from the fibre where it is bent. In European patent application 0211537A, there is described an improvement of a local launch and detect technique for use in aligning fibres prior to splicing, in which an optical fibre is optically coupled to an optical detector or a light source at the site of a bend in the fibre. The fibre is bent around a curved mandrel and, in the case of optical detection, light emitted from the bent fibre is passed by a resiliently deformable optical coupler, confined by a rigid transparent body, to a detector. Since the primary purpose of the technique is in pre-splice fibre alignment, there is no necessity for the coupling apparatus to exhibit a low insertion loss. Indeed since the coupling apparatus is said to be effective for the local launching of light, it appears likely that it exhibits a high insertion loss, it having been found that tight bending radii are required to permit the local launching of useful levels of light into a clad fibre. Moreover, the inefficiency of the optical coupling of the optical fibre to the detector is such that a small bend radius is needed to ensure detection with a conventional detector. Consequently, such a device would be unsuitable for use on a `live` fibre in a telecommunications system, where non-intrusive coupling is required. The maximum limit on insertion loss for non-intrusive systems is typically about 3 dB. Devices having insertion losses in excess of this figure may exceed the transmission system's operating margin and hence cause an unacceptable increase in bit rate error.
A further known optical coupling device which uses the bend loss phenomenon is described in Japanese patent application 58-188668. A fibre is again bent around a curved mandrel and emitted light is guided to a detector by means of a plurality of further optical fibres. These further optical fibres are embedded in a block of material against a curved face of which the fibre under test is pressed by the mandrel. The `collecting` fibres each have an end flush with the curved face of the block and are aligned so that in use they each extend orthogonally from the respective adjacent portion of the surface of the fibre under test. The use of a plurality of `collecting` fibres is said to lead to efficient detection of the leaked light.
There is again the disadvantage that the fibre under test must be exposed to a small bend radius, since it is only in this way that the requisite levels of emitted light will be coupled into the `collecting` fibres. Hence it is likely that the insertion loss of this device will also be too high to permit non-intrusive measurements to be made on `live` fibres. This device also suffers from non-optimum coupling of the test fibre to the detector, necessitating the use of a small bend radius.